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Background  Multi-drug resistance of Gram-negative bacteria constitutes a major obstacle in the antibacterial fight worldwide. The discovery of new and effective antimicrobials and/or resistance modulators is necessary to combat the spread of resistance or to reverse the multi-drug resistance. In this study, we investigated the antibacterial and antibiotic-resistance modifying activities against 29 Gram-negative bacteria including multi-drug resistant (MDR) phenotypes of the methanol extracts from  Nauclea pobeguiinii  leaves (NPL),  Nauclea pobeguiinii  bark (NPB) and six compounds from the bark extract, identified as 3-acetoxy-11-oxo-urs-12-ene ( 1 ),  p -coumaric acid ( 2 ), citric acid trimethyl ester ( 3 ), resveratrol ( 4 ), resveratrol  β-  D  - glucopyranoside ( 5 ) and strictosamide ( 6 ).    Methods  The broth microdilution method was used to determine the minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) of crude extracts and compounds as well as the antibiotic-resistance modifying effects of MPB and  4 .    Results  MIC determinations indicate values ranging from 32-1024 μg/mL for NPB and NPL on 89.7 % and 69.0 % of the tested bacterial strains respectively. MIC values below 100 μg/mL were obtained with NPB against  Escherichia coli  ATCC10536, AG100 and  Enterobacter aerogenes  CM64 strains. The lowest MIC value for crude extracts of 32 μg/mL was obtained with NPB against  E. coli  ATCC10536. Compound  4  was active all tested bacteria, whilst  1 ,  3  and  6  displayed weak and selective inhibitory effects. The corresponding MIC value (16 μg/mL) was obtained with  4  against  Klebsiella pneumoniae  KP55 strain. Synergistic effects of the combination of NPB with chloramphenicol (CHL), kanamycin (KAN) as well as that of compound  4  with streptomycin (STR) and ciprofloxacin (CIP) were observed.    Conclusion  The present study provides information on the possible use of  Nauclea pobeguinii  and compound  4  in the control of Gram-negative bacterial infections including MDR phenotypes. It also indicates that NPB and  4  can be used as naturally occurring antibiotic-resistance modulators to tackle MDR bacteria.    Electronic supplementary material  The online version of this article (doi:10.1186/s12906-016-1173-2) contains supplementary material, which is available to authorized users.

bmc complementary and alternative medicine

Antibacterial and antibiotic-resistance modifying activity of the extracts and compounds from Nauclea pobeguinii against Gram-negative multi-drug resistant phenotypes